In order to construct a preliminary model of control of growth and development of the testes a photoperiodic species, Zonotrichia leucophrys gambelii, will be subjected to light regimes selected to produce a variety of testicular growth rates. Pituitary LH, plasma LH, plasma testosterone and DHT, and testicular growth rate will be measured. Measurements will also be made through the course of the photorefractory period when testicular growth cannot be induced by photoperiodic treatment. The Bunning hypothesis and alternative coincidence models for time measurement in photoperiodically induced testicular growth will be tested by measurement of plasma concentration following single light perturbations.